by David Regal
When the magician notices that he's missing a few cards from his blue-backed deck, a few cards are randomly chosen by a spectator from a red-backed deck and added to the blue deck. When the magician discovers the missing blue-backed cards, they match the red-backed cards chosen by the spectator!
An ultra clean prediction of three cards. The prediction can be in an envelope from the start. It can be sent to a spectator a week ahead of time. The prediction can even be "open", left in full view before you begin.
Magicians have come to David's Magic Castle show a second and third time to try and figure out how this trick is done. Interestingly, the predicted cards can easily be made to change with each performance, frying the minds of those witnessing the effect for a second time. A subtle use of a principle.